Instrumentation systems for use in measuring industrial process variables such as flow, pressure, temperature, and liquid level typically employ a sensing element located in a field location adjacent the process which responds directly to the process variable. The output signal of the sensing element is transmitted to a distant central station, e.g., a control room, for further signal conditioning and processing. In the majority of present industrial applications, an electrical measurement signal is produced at the sensor, and a two-wire transmission line provides the interconnection necessary to power the sensor and receive the measurement signal.
One class of measurement instrument for developing such a measurement signal that has been known for many years employs resonant elements as the primary sensing device. More recently an accurate, practical family of instruments of this general type has been devised and successfully marketed by The Foxboro Company as its 800-Series resonant wire sensors. While these devices represent a significant advance as evidenced by the high degree of commercial success which they have obtained, they do possess certain limitations, particularly when operating in severe, highly electrically noisy process environments.
Thus, room for improvement exists in the design and construction of industrial measurement instruments, especially in their accuracy while operating within troublesome process environments, by eliminating or minimizing undesired electrical effects.